1. Field of the Invention:
The invention relates to bobbin thread tensioning devices for use in lockstitch sewing machines.
2. Description of the Prior Art:
The present invention is an improvement upon a bobbin thread tensioning device of the kind disclosed in U.S. Pat. No. 3,693,565 of Stanley J. Ketterer, for use in a lockstitch sewing machine wherein thread extends from a bobbin spool through the tension device at the peripheral surface of a bobbin case which supports the spool, passes under the hook, and finally emerges from an aperture in the throat plate to form stitches with needle thread in material being sewn. It is possible to set up such a machine to sew with a previously wound bobbin by manually drawing thread from the bobbin spool through the tensioning device, causing a length of the thread to be moved with the hook beak to a position partially around and partially under the hook as the hook is slowly rotated by the handwheel, and causing the hook beak with continued rotation of the handwheel to seize the needle thread and concatenate it around the bobbin thread such that the bobbin thread is moved by the needle thread wholly under the hook and up through the needle aperture in the throat plate. However, the construction of the tensioning device disclosed in said patent is such that bobbin thread exiting from the device isn't conveniently disposed for seizure by the hook beak. Unless the operator very carefully positions the loose end of the bobbin thread for seizure after pulling it through the tensioning device, the hook beak may fail to pick up and move the thread toward the underside of the bobbin case. Even with the bobbin thread properly positioned for seizure of the bobbin thread, seizure cannot be counted upon unless the hook beak is machined to close tolerances such as to cause it to enter a loop of bobbin thread. Furthermore, the construction of the tensioning device of the aforementioned patent requires removal of the bobbin case from the hook whenever bobbin thread tension is to be adjusted.